1. Field of the Invention
The present invention relates to a polymerization catalyst for use in producing a high molecular weight polyorganosiloxanes by polymerization of a low molecular weight organosiloxane.
2. Description of the Prior Art
Siliconates of quaternary phosphonium hydroxides are known as catalysts for polymerization of organosiloxanes, and have such properties that they exhibit catalytic activity at low temperatures and, when heated, readily decompose to loose the activity.
As contrasted to those catalysts which cannot be deactivated without neutralization, the above-mentioned siliconate catalysts can be used without need for removal of salts arising from neutralization and are known to be particularly useful for polymerization of crude rubbers or high-viscosity oils (See, for instance, J. Polymer Sci. 40. 35 (1959)).
Methods for producing siliconates of quaternary phosphonium hydroxides are disclosed, for instance, in U.S. Pat. No. 2,883,366, J. Polymer Sci. 40, 35 (1959), and so on. For example, a siliconate of a quaternary phosphonium hydroxide is known to be synthesized by dehydration condensation of an aqueous quaternary phosphonium hydroxide solution and a siloxane.
However, the quaternary phosphonium hydroxide siliconate catalysts produced by the conventional methods as mentioned above may show, during storage thereof, precipitation of crystals which are rich in the quaternary phosphonium hydroxide (the concentration of the quaternary phosphonium hydroxide in the crystals is from 45 to 55% by weight). Especially as the atmospheric temperature is lowered, the precipitation of the crystals takes place more heavily. The precipitated crystals collect, for example, in the bottom of a storage vessel, resulting in a variation of the concentration of the quaternary phosphonium hydroxide with the vertical position in the container.
Such a variation in the concentration of the quaternary phosphonium hydroxide is extremely undesirable because when the quaternary phosphonium hydroxide siliconate catalyst is used as a polymerization catalyst, the amount of the catalyst is reduced, in effect, and unsatisfactory polymerization would be caused thereby.
Once the aforementioned crystal precipitation has occurred, it is difficult to render the concentration of the quaternary phosphonium hydroxide uniform. For instance, a method of stirring the crystals and the liquid to obtain a uniform system, a method of heating the crystal-containing mixture to dissolve the crystals, and the like methods can produce a uniform quaternary phosphonium hydroxide concentration only momentarily, and the uneven concentration will soon appear again.